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According the the president of the Bonnyrigg High School P&C, the teenager was a 'normal' kid who had never been in any trouble.

Howard Chen emerged from a meeting with the school's principal on Thursday afternoon and said teachers were shocked by the brutal incident.

'He's like normal until Year 11 and suddenly this,' Mr Chen told Daily Mail Australia, describing the boy as being 'fine, all fine...This was a surprise.'

He said the principal Michael Bryce had told him teachers 'never have trouble' with the teenager, who, Mr Chen said, has twin brothers at the school.

Mr Chen said something must have changed 'just overnight, suddenly' for the boy, who had just returned for the first term of Year 11 when the incident allegedly happened.

The black Adidas back pack was lying in the car park next to a huge meat cleaver, a utility blade and an empty water bottle

A 15-year-old female student was stabbed in the shoulder in the terrifying attack and is in a stable condition in hospital

A teenage boy, 16, was treated on the scene by paramedics for stab wounds to his chest before being loaded into an ambulance and transported to hospital

According to a student, Ms Cox tried to stop the 16-year-old from attacking the others, but 'got caught in the crossfire'

Photos show a handcuffed student being escorted out of the school by police to a waiting police car

Two massive knives and a black backpack were discarded in the car park following the alleged attack on students

Police were called to Bonnyrigg High School at 9.15am on Thursday and a student was arrested at the scene

Forensic detectives were seen handling the weapons at in the car park including the meat cleaver (pictured)

A forensic investigator is seen inspecting the meat cleaver found in the car park

The area was sectioned off as police investigated the stabbing

Science teacher Carolyn Cox, 48, (pictured) suffered stab wounds to her back after allegedly trying to stop the 16-year-old attacking the others

The girl, 15, was stabbed in the shoulder and the male student, 16, suffered stab wounds to his chest and are both stable. There were at least four ambulances on the scene to help treat the victims.

A teenager was chased by police to a nearby Aldi supermarket and pictures show him being walked back to the car park with a number of detectives.

At one point a police officer drew his taser, but officers managed to arrest the teen before it was used.

A student, who was in the adjoining classroom at the time, told Daily Mail Australia the 16-year-old 'just lost it'.

She said Ms Cox allegedly tried to stop the student from attacking the others, but 'got caught in the crossfire'.

Another student shared a picture to Snapchat of police cordoning off the classroom where the stabbing allegedly took place. It is unknown how many students were in the room at the time and witnessed the attack.

Just after 11.30am, Superintendent Peter Lennon addressed the media and said while police had seized three knives from outside, they could find more weapons in the classroom once the crime scene is opened.

He said the incident was not terror related and the teenager, who lives locally, is not known to NSW police.

'This comes as a surprise to everybody,' he said.

The 16-year-old boy suffered stab wounds to his shoulder after another student 'just lost it'

According to Howard Chen (pictured), the president of the Bonnyrigg High School P&C, the teenager was a 'normal' kid who had never been in any trouble

A carving knife was also on the pavement near the back pack as the teenager was arrested

A forensic investigator picked up the huge knife while examining the scene on Thursday afternoon

A teenager was arrested outside an Aldi supermarket (pictured) next to the school and a number of people were seen milling outside hours after the attack

Students are seen outside at Bonnyrigg high school on Thursday

Detectives placed a blue sheet on the ground in the car park underneath the Adidas backpack to examine the scene

The meat cleaver, scissors and utility blade were analysed by the forensic team before being removed for further investigations

Officers are seen standing near evidence in the Aldi car park

An Ambulance NSW spokesman said the two Year 11 patients were treated at the high school before being taken to hospital in a stable condition.

Ms Cox's injuries are also not life-threatening and she is recovering at Westmead Hospital after being stabbed in the upper back.

The 16-year-old student is in police custody at Fairfield Police Station.

Just before 10am, the school confirmed lockdown was over and classes has returned to normal.

A number of terrified parents have rushed to the school to take their children home early after being told of the incident.

Mr Chen was glowing in his praise of Bonnyrigg High - saying he had a boy in the same year as the accused attacker and another son who had graduated and was studying at Macquarie University. 'It is the best school,' he said.

Students recalled scenes of panic as ‘alarms went off and bells rung’ early this morning. 'It was crazy,' one said.

Officers are investigating the scene and the Department of Education is setting up welfare services for students.

The girl, 15, and boy, 16, were both stabbed in the shoulder and there were at least four ambulances on the scene

Gloved officers held pens and notebooks as they searched the scene for hours after the attack on Thursday

The carving knife laid in the car park a few metres away from the back pack and meat cleaver

Just before 10am, the school confirmed lockdown was over and classes has returned to normal

Passerbys were told to remain outside the school gates as the ambulance pulled in on Thursday morning

There were at least four ambulances in the car park of the school before 10am on Thursday

Bonnyrigg High School is located in Sydney's west and provides placement for selective and international students, according to its website